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The development of well-defined transition metal catalysts for the selective cleavage of carbon-oxygen bonds is a critical challenge for the conversion of lignin to useful chemicals. Oxophilic titanocene complexes are particularly attractive for this application. They have been thoroughly studied in a variety of oxidation states and their reactivity can be dramatically altered by simple modifications of the ancillary ligands. Additionally, titanium is earth-abundant, inexpensive, and relatively non-toxic.
Titanocene-mediated carbon-oxygen bond cleavage of the α-aryloxy ketones 2-phenoxy-1-phenylethanone and 2-(2,6-dimethoxyphenoxy)-1-phenylethanone has been demonstrated. In contrast to previously reported C-O bond cleavage reactions of lignin model compounds, this transformation occurs readily at ambient temperature and pressure.
A titanocene phenoxide enolate complex was obtained as a result of the C-O bond cleavage of 2-phenoxy-1-phenylethanone. Selective cleavage of the Ti-enolate bond was achieved with the use of mild acids. Alternatively, both products of the C-O bond cleavage, acetophenone and phenol, can be released from titanium through the use of strong acids, without displacing the cyclopentadienyl ligands.
Titanocene aryloxide enolate complexes with ortho-substituted aryloxy ligands were synthesized. A weakening of the Ti-OAr bond was observed as a result of the increased steric bulk. This allowed for partial or complete protonolysis of both Ti-O bonds with the use of the mild acid lutidinium chloride.
A titanocene alkoxide phenoxide complex was prepared. The elimination of phenol from this compound was expected to lead to the formation of a titanaoxirane intermediate, which would then undergo C-O bond cleavage to yield titanocene phenoxide enolate. This reaction is an essential step in the target catalytic cycle for the C-O bond cleavage of lignin model compounds. However, the desired transformation could not be achieved and related titanocene complexes with substituted aryloxy ligands could not be synthesized.
The use of zirconocene was investigated to alleviate to some of the problems encountered with titanocene chemistry. In particular, the ability of zirconium to form 18-electron complexes resulted in different reactivity as compared to titanocene.

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